Compositions for oxide CMP

ABSTRACT

A chemical mechanical polishing composition comprising a soluble cerium compound at a pH above 3 and a method to selectively polish a silicon oxide overfill in preference to a silicon nitride film layer in a single step during the manufacture of integrated circuits and semiconductors.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is divisional of co-pending U.S. patentapplication Ser. No. 08/994,894, filed on Dec. 19, 1997, which is acontinuation-in-part of U.S. patent application Ser. No. 08/774,488filed on Dec. 30, 1996, now U.S. Pat. No. 5,759,917, issued on Jun. 2,1998.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to chemical mechanical polishingslurries for semiconductor integrated circuit substrates. Specifically,this invention is a CMP slurry having a unique chemistry that isespecially suitable for chemical mechanical planarization where a highsilicon dioxide removal rate and a low silicon nitride removal rate arerequired on the same substrate.

[0004] 2. Description of the Related Art

[0005] Integrated circuits (IC) are made up of millions of activedevices formed in or on a silicon substrate. The active devices formfunctional circuits and components. These devices are then connected bythe use of multilevel metallized interconnects and vias. Interconnectionstructures normally have a first layer metallization, an interconnectplug, a second layer of metallization, and sometimes a third or morelayers of metallization with their respective interconnects. Inter leveldielectrics (ILDs), such as doped and undoped SiO₂, are used toelectrically isolate the different levels of interconnections.

[0006] Shallow trench isolation (STI) is a technology for deviceisolation in a give layer in the IC manufacturing process. In the STIprocess, silicon nitride is deposited on thermally grown oxide. Afterdeposition of the nitride, a shallow trench is etched into the substrateusing a mask. A layer of oxide is then deposited into the trench so thatthe trench forms an area of insulated dielectric that acts to isolatethe devices in a chip and, thus, reduces the cross-talk between activedevices. The excess deposited oxide must be polished off and the trenchplanarized to prepare for the next level of metallization. Siliconnitride is applied to the silicon to prevent polishing of the maskedsilicon oxide of the device.

[0007] In a typical mechanical polishing process, the substrate isplaced in direct contact with a rotating polishing pad. A carrierapplies pressure against the backside of the substrate. During thepolishing process, the pad and table are rotated while a downward forceis maintained against the substrate back. An abrasive and chemicallyreactive solution, commonly referred to as “CMP slurry”, is flowed ontothe pad during polishing. The chemicals and abrasive particles in theslurry initiate the polishing process by interacting with the waferbeing polished. As slurry is provided to the wafer/pad interface, andthe polishing process is facilitated by the rotational movement of thepad relative to the substrate. Polishing is continued in this manneruntil the final desired film thickness is achieved by removal of therequired amount of thin-film material.

[0008] When polishing oxides, it is desirable that the slurry have ahigh removal rate towards the oxide layer and a low removal rate towardsother layers that may be exposed during CMP. A polishing slurry shouldbe tailored to provide effective polishing at the desired polishingranges selective to specific thin layer materials, while at the sametime minimizing surface imperfections, defects, corrosion, erosion andthe removal of silicon nitride and other stop layers.

[0009] CMP slurries useful for polishing oxides typically contain anabrasive at an alkaline or high pH. These slurries either rely onpotassium hydroxide or ammonium hydroxide to effectively buffer the highpH. While these slurries polish silica at high rates, they also polishsilicon nitride at high rates. Typically, the ratio of these removalrates, i.e., the selectivity is, at most, about 5 to 1 silicon oxide tosilicon nitride. It is believed that the mechanism of silicon nitridepolishing is oxidative hydrolysis of the nitride to the oxide in anaqueous environment. At an alkaline pH this oxide and nitride aresimilarly etched at a high rate. Thus, present CMP slurries undesirablypolish silicon nitride at an unacceptably high rate.

[0010] There remains a need in the semiconductor industry for CMPslurries that have greater than a 5 to 1 oxide to nitride selectivityratio. Accordingly, new CMP slurries that selectively remove the oxideat high rates while leaving the stop layer of silicon nitride relativelyintact, while increasing the output and reducing the costs of the CMPprocess are needed to overcome the present manufacturing problems. Thisis because a low selectivity process, when used in a manufacturingenvironment, will necessarily suffer overpolishing—in thinner film partsof the wafer—and the nitride stop layer will not prevent breakthrough tothe underlying thin film(s).

SUMMARY OF THE INVENTION

[0011] This invention is a chemical mechanical polishing compositionthat is capable of polishing a silicon dioxide layer at a high rate.

[0012] This invention is also a chemical mechanical polishingcomposition that inhibits the polishing of a silicon nitride film.

[0013] In addition, this invention is a method of using a chemicalmechanical polishing composition that selectively removes silicondioxide from a substrate while leaving a silicon nitride layerassociated with the substrate essentially intact.

[0014] In one embodiment, this invention is a chemical mechanicalpolishing composition comprising carboxylic acid, a salt and a solublecerium compound. The composition has a pH of from about 3.0 to about 11,and preferably from about 3.8 to about 5.5 and is useful for selectivelyremoving silicon dioxide from layered substrates.

[0015] In another embodiment, this invention is a chemical mechanicalpolishing slurry comprising the chemical mechanical polishingcomposition described above and an abrasive. The slurry is especiallyuseful for silicon dioxide film polishing.

[0016] In still another embodiment, the present invention is a methodfor using a chemical mechanical polishing composition comprising acarboxylic acid, a salt and a soluble cerium compound in an aqueoussolution having a pH of from about 3.0 to about 11 to selectively removeoxide overfill in preference to a silicon nitride film layer during themanufacture of integrated circuits and semiconductors.

BRIEF DESCRIPTION OF THE DRAWING

[0017]FIG. 1—Plot of pH versus PETEOS removal rate and nitrate removalrate.

DESCRIPTION OF THE CURRENT EMBODIMENTS

[0018] The present invention is directed to a chemical mechanicalpolishing composition that comprises a carboxylic acid, a salt, and asoluble cerium compound, having a pH of from about 3.0 to about 11.0.The chemical mechanical composition may be used alone or it may becombined with a metal oxide abrasive to form a slurry. The compositionsand slurries of this invention polish oxide layers such as silicondioxide layers associated with substrates at high rates. In addition,the compositions of this invention have been found to inhibit siliconnitride polishing. The present invention is also directed to novelmethods for using the compositions and slurries of this invention topolish oxide layers.

[0019] Before describing the details of the various preferredembodiments of this invention a phrase that is used herein will bedefined. “Chemical mechanical composition” refers to the combination ofat least one carboxylic acid, at least one salt, and at least onesoluble cerium compound that may be used in conjunction with an abrasivepad to remove one or more layers of a substrate. The term “slurry” andthe phrase “chemical mechanical polishing slurry” refer to thecombination of the chemical mechanical polishing composition and atleast one abrasive.

[0020] Carboxylic acids useful in a CMP slurry of the present inventioninclude monofunctional and di-functional carboxylic acids and theirsalts. Preferably, the carboxylic acid is selected from the groupincluding acetic acid, adipic acid, butyric acid, capric acid, caproicacid, caprylic acid, citric acid, glutaric acid, glycolic acid, formicacid, fumaric acid, lactic acid, lauric acid, malic acid, maleic acid,malonic acid, myristic acid, oxalic acid, palmitic acid, phthalic acid,propionic acid, pyruvic acid, stearic acid, succinic acid, tartaricacid, valeric acid, 2-(2-methyoxyethoxy) acetic acid,2-[2-(2-methyoxyethoxy)ethyoxy] acetic acid, poly(ethyleneglycol)bis(carboxymethyl)ether, and derivatives, including saltsthereof. A most preferred carboxylic acid is acetic acid.

[0021] In the composition of the present invention, the carboxylic acidcan comprise greater than 10% of the slurry. In a preferred embodiment,the carboxylic acid is present in the composition of this invention inan amount ranging from about 0.05 to about 10% by weight. In a morepreferred embodiment, however, the carboxylic acid is present in thecomposition of this invention in an amount ranging from about 0.1 toabout 3%.

[0022] The chemical mechanical composition of the present invention mayinclude a salt. The term “salt” refers to any water soluble saltsincluding organic salts and inorganic salts such as nitrate, phosphate,and sulfate salts. “Soluble salts” also refers to salts that are onlypartially or marginally soluble in water. Preferred salts are nitratesalts.

[0023] The phrase “nitrate salt” includes nitric acid. Useful nitratesalts include compositions of the formula (M)_(n)(NO₃)_(m) where n and mare both integers. When n=m, M is monovalent and can be alkali earthmetals such as Li, Na, K, as well as H, NH₄, NR₄ where R is an alkylgroup having from 1 to 10 or more carbon atoms or is a mixture thereof,including NMe₄, NBu₄ and so forth. When n≠m, then M is a multivalentcation or metal or a combination of a multivalent cations and monovalentcations. One known preferred nitrate salt is ammonium cerium nitrate,(NH₄)₂Ce(NO₃)₆.

[0024] The salt may be present in the composition in the amount of fromabout 0.05 to about 6% by weight of the composition. It is mostpreferred that the salt is present in the composition in the amountranging from about 0.1 to about 4% by weight.

[0025] The chemical mechanical composition of the present inventionincludes at least one soluble cerium compound. The phrase “solublecerium” includes, for purposes of this invention, both cerium added insoluble form and cerium dissolved from colloidal or ground particles.Non-limiting examples of soluble cerium compounds useful in acomposition of the present invention include water soluble hydrated andnon-hydrated salts of cerium hydroxide (Ce(OH)₄), ammonium ceriumsulfate, (NH₄)₂SO₄Ce₂(SO₄)₃; cerium acetate, Ce(O₂CH₃)₃; cerium sulfate,Ce(SO₄)₂; cerium bromate, Ce(BrO₃)₃.9H₂O; cerium bromide, CeBr₃; ceriumcarbonate, Ce(CO₃)₂, cerium chloride, CeCl₃; cerium oxalate, Ce(C₂O₄)₃;cerium nitrate, Ce(NO₃)₃(OH).6H₂O and any other known soluble ceriumcompounds. A preferred soluble cerium compound is ammonium ceriumnitrate, (NH₄)₂Ce(NO₃)₆. The soluble cerium compound will be present inthe composition of this invention in an amount ranging from about 0.05wt. % to about 10.0 wt. % and preferably from about 0.1 to about 4.0 wt.%.

[0026] A preferred embodiment of the chemical mechanical composition ofthe present invention includes ammonium cerium nitrate as both the saltand as the soluble cerium compound. Other soluble cerium nitrate saltsmay be incorporated into the composition of this invention as both thesoluble cerium compound and as the salt. Ammonium cerium nitrate may bepresent in composition of the present invention in an amount rangingfrom about 0.05 to about 6% wt. % of the overall composition weight. Amore preferred range of ammonium cerium nitrate is from about 0.1 toabout 4.0 wt. %.

[0027] Commercially available colloidal ceria typically contains amixture of dissolved Ce⁴⁺ and Ce³⁺ ions. It is preferred that thedissolved ceria is in the form of Ce³⁺ ions. Adding an oxidizer to thecomposition of this invention that is capable of oxidizing Ce³⁺ to Ce⁴⁺produces a product that exhibits high oxide selectivities and lownitride selectivities. The oxidizing agent used must have a higheroxidation potential than Ce⁴⁺. A preferred oxidizing agent is ammoniumpersulfate. The oxidizing agent is useful in an amount ranging fromabout 0.05 to about 5.0 wt. %. The oxidizing agent is preferably presentin an amount ranging from about 0.1 to about 2.0 wt. %.

[0028] The chemical mechanical composition of this invention mayoptionally include at least one chelating agent. It has been found thatthe addition of a chelating agent to chemical mechanical compositions ofthis invention improves the cleanability of substrates polished withcompositions of this invention. It is believed that cleanability ofsubstrates is enhanced because the chelating agent binds free ions inthe composition that would otherwise deposit on the wafer.

[0029] Useful chelating agents include any chelating agents that bind tofree ions in the compositions of this invention. Examples of usefulchelating agents include, but are not limited, to polycarboxylic acidssuch citric acid, EDTA, triethanolamine, and benzonitrile, adipic acid,malonic acid, oxalic acid, phosphonic acids, phosphoric acid and saltsthereof. Chelating agents, if used, should be present in the compositionin an amount ranging from about 0.05 to about 5.0 wt. %, and preferablyfrom about 0.1 to about 1.5 wt. %.

[0030] The chemical mechanical composition of this invention may be usedalone or in conjunction with an abrasive to give a chemical mechanicalpolishing “slurry.” Abrasives useful in conjunction with thecompositions of the present invention include metal oxide abrasives. Themetal oxide abrasive may be selected from the group including alumina,titania, zirconia, germania, silica, ceria and mixtures thereof. Inaddition, useful abrasives may be the result of mixing precursors of twoor more metal oxides to give a chemical admixture of a mixed metal oxideabrasive. For example, alumina can be co-formed with silica, or combinedalumina/silica.

[0031] Useful metal oxide abrasives may be produced by any techniquesknown to those skilled in the art, including high temperature processessuch as sol-gel, hydrothermal or, plasma process, or by processes formanufacturing fumed or precipitated metal oxides. Pulverized or crushedmetal oxide abrasives are also useful in the CMP slurry of thisinvention and may be manufactured by milling or grinding usingconventional manufacturing techniques such as jet-milling, ball milling,bead milling, and other milling and pulverizing techniques and processknown to one skilled in the art.

[0032] Preferred abrasives suitable for the CMP slurries of thisinvention are silica and cerium oxide (ceria) with fumed silica beingmost preferred. Other suitable silica abrasives can be made by methodssuch as sol-gel, hydrothermal, plasma process, flame pyrolysis or byother processes for manufacturing metal oxides.

[0033] Pulverized abrasives are also suitable for this invention. Anypulverized metal oxide abrasive may be used in a CMP slurry of thisinvention. However, pulverized cerium oxide is preferred. The ceriumoxide abrasive is ground in a media mill to give pulverized ceria. Theoriginal cerium oxide particle may be either mined cerium oxide orprecipitated and calcined cerium oxide or a combination thereof. Thegrinding may be accomplished in an aqueous medium using any type of agrinding or milling apparatus such as by jet milling or ball milling. Apreferred grinding mechanism is a medial mill with either yttriatetragonal zirconia (YTZ) or zirconium silicate media. The grindingprocess may use a dispersant or steric stabilizer.

[0034] The preferred pulverized metal oxide abrasive will have a narrowparticle size distribution with a median particle size (i.e., aggregateparticle or single particle) of less than about 0.5 microns. Theparticles may be diluted and filtered after the grinding. Preferably,after filtration, the particle sizes of the pulverized metal oxideabrasive range from about 40 to about 1000 nm, and preferably from about100 to about 300 nm. The preferred pulverized abrasive should containless than about 10 wt. % of particles having a median particle sizegreater than 0.6 μm.

[0035] Precipitated cerium oxide is a suitable abrasive for oxide CMP.Precipitated cerium oxide particles are made from a variety ofprecursors including acetates, carbonates and hydroxide and nitratesalts of cerium. The median particle size of precipitated cerium oxideparticles may range of from about 10 nm to about 500 nm, with thepreferred size of precipitated cerium oxide particles being in the rangeof from about 30 to about 300 nm.

[0036] Another preferred abrasive is fumed silica. The production offumed metal oxides is a well-known process that involves the hydrolysisof suitable feed stock vapor (such as silicon tetrachloride for a silicaabrasive) in a flame of hydrogen and oxygen. Molten particles of roughlyspherical shape are formed in the combustion process. The diameters ofthe particles are varied through process parameters, and these moltenspheres of silica or similar oxide, typically referred to as primaryparticles, fuse with one another by colliding at their contact points toform branched, three dimensional chain-like aggregates. The forcenecessary to break aggregates is considerable and often irreversible.During cooling and collecting, the aggregates undergo further collisionsthat may result in some mechanical entanglement causing the formation ofaggregates.

[0037] A preferred metal oxide will have a surface area, as calculatedfrom the method of S. Brunauer, P. H. Emmet, and I. Teller, J. Am.Chemical Society, Volume 60, Page 309 (1938) and commonly referred to aBET, ranging from about 5 m²/g to about 430 m²/g and preferably fromabout 30 m²/g to about 170 m²/g. Due to stringent purity requirements inthe IC industry the preferred metal oxide should be of a high purity.High purity means that the total impurity content, from sources such asraw material impurities and trace processing contaminants, is typicallyless than 1% and preferably less than 0.01% (i.e., 100 ppm).

[0038] In a preferred embodiment, the metal oxide abrasive consists ofmetal oxide aggregates having about 99 wt. % of the particles less thanabout 1.0 micron in diameter, a mean aggregate diameter less than about0.4 micron and a force sufficient to repel and overcome the van derWaals forces between abrasive aggregates themselves. Such metal oxideabrasives have been effective in minimizing or avoiding scratching, pitmarks, divots and other surface imperfections during polishing. Theaggregate size distribution in the present invention may be determinedusing known techniques such as transmission electron microscopy (TEM).The mean aggregate diameter refers to the average equivalent sphericaldiameter when using TEM image analysis, i.e., based on thecross-sectional area of the aggregate. The surface potential or thehydration force of the metal oxide particles must be sufficient to repeland overcome the van der Waals attractive forces between the particles.

[0039] In another preferred embodiment, the metal oxide abrasive mayconsist of discrete metal oxide particles having a particle diameterless than 0.5 micron (500 nm) and a surface area ranging from about 10m²/g to about 250 m²/g.

[0040] A CMP slurry of this invention will include from about 2 wt. % toabout 25 wt. % metal oxide abrasive and preferably from about 2 wt. % toabout 15 wt. % metal oxide abrasive.

[0041] Metal oxide abrasives useful in CMP slurries of the presentinvention are incorporated into the aqueous medium of the polishingslurry as a concentrated aqueous dispersion of metal oxides comprisingfrom about 3% to about 55% solids, and preferably between 30% and 50%solids. The aqueous dispersion of metal oxides may be produced usingconventional techniques, such as slowly adding the metal oxide abrasiveto an appropriate media, for example, de-ionized water, to form acolloidal dispersion. The dispersions are typically completed bysubjecting them to high shear mixing conditions known to those skilledin the art.

[0042] The abrasives useful in a CMP slurry of the present invention canbe a mixture of the abrasives described above. For example, precipitatedcerium oxide, pulverized cerium oxide (also referred to a ceria) andfumed silica could be incorporated into a CMP slurry of the presentinvention. Other combinations of abrasives are also useful in the CMPslurry. In addition, the mixture of abrasives could include any relativeproportion of one abrasive to another. For example, a combination offrom about 5 to 100 wt. % of the pulverized oxide abrasive describedabove with from about 0 to about 95 wt. % precipitated abrasive has beenfound to be effective as a CMP slurry abrasive in STI applications.

[0043] Commercially available precipitated cerium oxides sold at a pH ofabout 1.5 are ineffective as CMP slurries. We have, however, discoveredthat significantly increasing the pH of the commercially availableslurry to about 3.5 results in a CMP slurry that is useful for STIpolishing. Furthermore, we have surprisingly discovered that a CMPslurry with the composition and pH disclosed above exhibits a high oxidelayer removal rate and low nitride layer removal rate.

[0044] The CMP slurry of this invention must have a pH of from about 3.0to about 11.0 to be effective. More preferably, the slurry pH will rangefrom about 3.5 to about 6.0, and most preferably the pH is from about3.8 to about 5.5. Slurry pH is adjusted by adding any base to thecomposition and preferably by adding a non-metal base such as ammoniumhydroxide to the slurry.

[0045] The chemical mechanical composition of this invention may containone or more buffering agents. The purpose of the buffering agent is tohelp maintain the pH of the composition within the desired range, andmost preferably from about 3.8 to about 5.5.

[0046] Any buffering agent that is capable of maintaining the pH of thecomposition in the desired range may be used. Most preferred buffers areammonium formate or formic acid. Buffering agent used in the compositionwill typically range from about 0.01 to about 5.0 wt. % and mostpreferably from 0.05 to about 0.5 wt. %.

[0047] In order to further stabilize a polishing slurry of thisinvention against settling, flocculation and decomposition of theoxidizing agent, a variety of additional optional additives, such assurfactants, polymeric stabilizers or other surface active dispersingagents, can be used. The surfactant can be anionic, cationic, nonionic,amphoteric and combinations of two or more surfactants can be employed.Furthermore, it has been found that the addition of a surfactant may beuseful to improve the within-wafer-non-uniformity (WIWNU) of the wafers,thereby improving the surface of the wafer and reducing wafer defects.

[0048] In general, the amount of an additive, such as a surfactant, usedin the present invention should be sufficient to achieve effectivesteric stabilization of the slurry and will typically vary depending onthe particular surfactant selected and the nature of the surface of themetal oxide abrasive. For example, if not enough of a selectedsurfactant is used, it will have little or no effect on stabilization.On the other hand, too much of the surfactant may result in undesirablefoaming and/or flocculation in the slurry. As a result, additives likesurfactants should generally be present in a range between about 0.001%and 10% by weight. Furthermore, the additive may be added directly tothe slurry or treated onto the surface of the metal oxide abrasiveutilizing known techniques. In either case, the amount of additive isadjusted to achieve the desired concentration in the polishing slurry.

[0049] The chemical mechanical polishing compositions and slurries ofthis invention are capable of selectively removing the silicon dioxidelayer from layered substrates at very high rates. Furthermore, thecompositions and slurries of this invention inhibit the polishing ofsilicon nitride from layered substrates. One important application forthe chemical mechanical polishing compositions and slurries of thisinvention is in the manufacture of integrated circuits andsemiconductors. In such a polishing application, the compositions andslurries of this invention effectively remove silicon dioxide forshallow trench device isolation.

[0050] The compositions and slurries of this invention preferablyexhibit oxide removal rates of from about 1200 Å/min to about 6000 Å/minor more with an oxide to nitride removal selectivity of from about 5 toabout 100 or more and preferably from about 15 to about 50 or more.

[0051] The compositions and slurries of the present invention may beincorporated in a single package that includes an aqueous composition ofat least one carboxylic acid, a soluble cerium compound, a salt, anoptional abrasive, and optional additives at the requisite pH. To avoidchanges in slurry performance over time it may be preferable to use atleast a two package system where the first package comprises at leastone carboxylic acid, a salt and a soluble cerium compound at any pH andthe second package comprises the optional abrasive at any pH. These twopackages will be engineered so that when they are mixed, the usefulcomposition is in the required pH range. Alternatively, the componentsin one container may be in dry form while the components in the othercontainer are in the form of an aqueous dispersion. Other two-containercombinations of the ingredients of the CMP slurry of this invention arewithin the knowledge of one having ordinary skill in the area.

[0052] At the requisite pH, the compositions and slurries of the presentinvention do not significantly increase the silicon nitride removalrate. However, they significantly increase the removal rate of silicondioxide in comparison to known slurries. The polishing slurries of thepresent invention may be used during the various stages of semiconductorintegrated circuit manufacture to provide effective removal of siliconoxide layers at desired removal rates while minimizing surfaceimperfections and defects.

EXAMPLES

[0053] The following examples illustrate preferred embodiments of thisinvention as well as preferred methods for using compositions of thisinvention. All compositions and slurries were used in an STI polishingprotocol as outlined below.

Example 1

[0054] The CMP slurries were used to chemically-mechanically polishblanket PETEOS and silicon nitride using an IC1000/SUBA IV pad stackmanufactured by Rodel, Inc. The polishing was performed using anIPEC/WESTECH 472 CMP tool at a down force of 9 psi, a slurry flow rateof 140 ml/min., a platen speed of 35 rpm and a carrier speed of 24 rpm.

Example 2 Pulverized Ceria Formulations

[0055] A pulverized ceria slurry was prepared in order to evaluate itsability to polish blanket silicon dioxide and nitride wafers. Rhoditegrade 400HS ceria, having particle sizes approximately 2-4 μm waspurchased from Universal Photonics, Hicksville, N. Y. and pulverizedusing an agitator bead mill to a primary median particle size of 150 nm.Pulverizing was accomplish under wet conditions so that the resultingslurry, pH approximately 7.5-8.5, contained 20-30% solids after thepulverizing process.

[0056] The slurry was then diluted and the pH adjusted to produce theslurries listed in Table 1. The slurries were used to polish substratesaccording to the method described in Example 1. TABLE 1 PETEOS RRNitride RR Slurry No. pH % solids (Å/min) (Å/min) Selectivity 1 8 4.0925 1050 0.89 2 8 5.0 4337 1137 3.81 3 8 7.5 4800 1130 4.25 4 8 10.05145 1153 4.46 5 10 4.0 4342 1101 3.95 6 10 10.0 4344 1015 4.28

[0057] The pulverized ceria slurries were used for polishing The dataindicates that the pulverized ceria slurries yield very high PETEOS(silicon oxide layer) removal rates.

Example 3 Precipitated Ceria Nitrate Formulations

[0058] A nitrate stabilized ceria slurry containing precipitated ceriaparticles, nitric acid, acetic acid, pH=1.8 and 20% solids, waspurchased from Nyacol Products (Ashland, Mass.). The pH of the slurrywas adjusted to from about 4.2-6.8 by adding ammonium hydroxide. Theslurries were used to polish substrates according to the methoddescribed in Example 1. The polishing results are reported in Table 2.TABLE 2 Slurry % PETEOS Nitride No. pH solids Additives RR RRSelectivity 7 4.2 20 406 14.5 28 8 5.8 20 281 208 1.35 9 6.1 20 241 2810.86 10 6.2 20 163 354 0.46

[0059] The polishing data indicates that at the lowest pH (4.2)selectivity is high, but overall oxide removal rates are low.

Example 4 Precipitated Ceria Acetate Formulations

[0060] A colloidal cerium acetate slurry, including soluble Ce³⁺ andCe⁴⁺, and acetic acid, (pH=0.8 and 20% solids), was purchased fromNyacol Products (Ashland, Mass.). The pH of the slurry was adjusted to4.5 and the solids content to 15%. The slurry was applied to a substrateaccording to the methods described in Example 1 and the result showed anoxide layer removal rate of 117 Å/min and a nitride layer removal rateof 10.5 Å/min for an oxide to nitride of 11.1.

Example 5 Crushed/Precipitated Ceria Formulations

[0061] A ceria slurry, composed of varying wt. % amounts of thepulverized ceria manufactured as set forth in Example 2 and precipitatedceria purchased from Nyacol Products (Ashland, Mass.) was formulated asshown in Table 3. The slurries were used to polish substrates accordingto the methods described in Example 1 and the polishing results are setforth in Table 3, below. TABLE 3 % pulverized PETEOS Slurry % totalceria in RR Nitride RR No. pH solids slurry (Å/min) (Å/min) Selectivity11 4 8 20 1595 108.4 14.71 12 4 8 40 2168 183.4 11.82 13 4 8 60 3356826.5 4.06 14 4 8 80 4785 209.1 22.88

[0062] The results indicate that a CMP slurry including 80% pulverizedceria and 20% precipitated ceria produced the most desired properties ofhigh PETEOS rates, low nitride rates and high selectivity.

Example 6 Chemical Formulation Using Pulverized Ceria

[0063] A slurry, composed of L-90, a fumed silica particles manufacturedby Cabot Corporation and sold under the trademark CAB-O-SIL®, ammoniumcerium nitrate, acetic acid, of varying percentages, and deionized waterwas formulated as shown in Table 4. All slurries were adjusted to pH=4after the inclusion of additives. The slurries were applied to substrateaccording to the methods described in Example 1. TABLE 4 Weight Weight %Amm. % Nitride PETEOS Weight % Cerium Acetic RR RR Selec- Slurry silicaNitrate Acid (Å/min) (Å/min) tivity 20 4 0.1 0.1 58 280 4.83 21 4 0.1 152 253 4.87 22 4 0.65 0.5 59 619 10.49 23 4 1 0.1 44 1535 34.89 24 4 1 1312 1524 4.88 25 4 1 0 104.62 1337.9 12.79 26 4 2 0.05 57.51 1103 19.1827 4 3 0.1 89.99 835.8 9.29 28 4 1 0.5 71.5 803.1 11.23 29 4 2 0.1 24.1346.6 14.38 30 4 2 0.5 71.1 768.0 10.8

[0064] High PETEOS removal rates and low nitride removal rates wereobtained with high nitrate (1% nitrate) content and low (0.1%) aceticacid content.

Example 7 Chemical Formulation Using Silica—pH Test

[0065] A slurry composed of 4 wt. % CAB-O-SIL® L-90 famed silica, 1.8wt. % ammonium cerium nitrate, and 0.6 wt. % acetic acid of varyingpercentages was formulated as shown in Table 5. The pH of the slurriesvaried from between 4.0 to 5.0. The slurries were applied to substrateaccording to the methods described in Example 1. TABLE 5 Weight Weight %Acetic PETEOS Slurry % silica pH Acid Nitride RR RR Selectivity 31 4 4.00.6 114 1713.7 15.03 32 4 4.3 0.6 141 1988.9 14.11 33 4 4.7 0.6 1992810.5 14.12 34 4 5.0 0.6 219 2355 10.75

[0066] High PETEOS removal rates are obtained and selectivity was verygood for each slurry. The results indicate that slurry pH has a strongeffect on PETEOS removal rate and the optimum removal rate of oxide isachieved at about pH 4.7 (FIG. 1).

Example 8

[0067] A composition composed of 1.8 wt. % ammonium cerium nitrate, 0.8wt. % acetic acid, and deionized water was used to polish PETEOS andsilicon nitride wafers according to the method of Example 1. The pH ofthe slurry was adjusted to 4.5. The composition polished PETEOS at 690Å/min and silicon nitride at 23 Å/min, giving a PETEOS selectivity of30.

Example 9

[0068] A CMP slurry consisting of 4.0 wt % cerium and 4.0 wt. % silica,having a pH of 4.5 was prepared by combining appropriate amounts of (1)the 20 wt. % colloidal cerium solution manufactured by Nyacol Products(Ashland, Mass.) and described in Example 4; (2) L-90 fumed silicamanufactured by Cabot Corp. and sold under the name CAB-O-SIL®; and (3)deionized water. The pH of the slurry was adjusted to 4.5 with ammoniumhydroxide. The slurry included 0.15 wt % ammonium persulfate.

[0069] Prior to testing, varying amounts of EDTA or citric acidchelating agents were added to the slurry. The slurries were then testedaccording to the method of Example 1. The test results are reported inTable 6, below. TABLE 6 Chelating Oxide Chelating Agent Removal NitridePost CMP clean Agent (wt %) rate Removal rate (LPD) EDTA-K 0 387019 >20,000 EDTA-K 0.1 2731 11 977 EDTA-K 0.2 1806 12 169 EDTA-K 0.3 138111 45 Citric Acid 0 4241 20 3772 Citric Acid 0.10% 2095 34 516 CitricAcid 0.20% 1625 68 28

[0070] Increasing the amount of dipotassium EDTA in the slurry decreasesthe oxide removal rate. However, increasing the dipotassium EDTAconcentration significantly decreases the Light Point Defects (LPD) thenumber of particles on the wafer after post CMP clean.

Example 10

[0071] This Example evaluated the polishing effectiveness ofcompositions of this invention without and with an oxidizing agent.Slurries, consisting of 4.0 wt. % colloidal ceria including Ce⁴⁺ ions,and 4.0 wt. % silica and having a pH of 4.5 were formulated according tothe method set forth in Example 9. The slurries were tested according tothe method of Example 2 without and with the addition of 0.15 wt %ammonium persulfate. The test results are reported in Table 7, below.TABLE 7 No Oxidizer With Oxidizer Oxide Nitride Oxide Nitride Rate RateRate Rate Lot (Å/min) (Å/min) Selectivity (Å/min) (Å/min) Selectivity 12822 472 5.98 3255 28.9 112.6 2 3394 373 9.10 3513 22.8 154.1 3 3640 31911.42 3428 25.8 132.9 4 2929 473 6.19 3711 36.1 102.8 5 1734 856 2.023880 46.5 83.4

[0072] The results set forth in Table 7 indicate that, without ammoniumpersulfate oxidizer, each lot of ceria exhibits different polishingrates and acceptable but low nitride selectivities. The performance ofthe same lots after the addition 0.15 wt % ammonium persulfate is moreconsistent and the slurries exhibit high nitride selectivities.

[0073] While the present invention has been described by means ofspecific embodiments, it will be understood that modifications may bemade without departing from the spirit of the invention. The scope ofthe invention is not limited by the description of the invention setforth in the specification and examples, but rather defined by thefollowing claims.

1. A method for removing at least a portion of a silicon dioxide layerfrom a substrate comprising: (a) admixing a salt, a soluble ceriumcompound including Ce⁴⁺ ions; an oxidizing agent having a oxidationpotential greater than Ce⁴⁺, and de-ionized water to give a chemicalmechanical polishing composition having a pH of from about 3.0 to about11.0; (b) applying the chemical mechanical polishing composition to thesubstrate; and (c) removing at least a portion of the silicon oxidelayer from the substrate by bringing a pad into contact with thesubstrate and moving the pad in relation to the substrate.
 2. The methodof claim 1, wherein the substrate is a layered substrate comprising atleast one layer of silicon dioxide and at least one layer of siliconnitride.
 3. The method of claim 1, wherein the silicon oxide is removedfrom the substrate at a rate at least five-fold greater than the removalrate of silicon nitride.
 4. The method of claim 1, wherein the oxidizingagent is ammonium persulfate.
 5. The method of claim 1, wherein the saltand soluble cerium compound is ammonium cerium nitrate.
 6. The method ofclaim 1, including at least one metal oxide abrasive selected from thegroup including alumina, titania, zirconia, germania, silica, ceria andmixtures thereof.
 7. The method of claim 1, wherein the metal oxideabrasive is silica.
 8. A method for removing at least a portion of asilicon dioxide layer deposited on a silicon wafer including a siliconnitride layer comprising: (a) mixing from about 2 to about 15 wt. %silica, from about 0.05 to about 10 wt. % ammonium cerium nitrate, fromabout 0.05 to about 5.0 wt. % ammonium persulfate, at least onechelating agent and de-ionized water to give a chemical mechanicalpolishing slurry having a pH of between about 3.8 to about 5.5; (b)applying the chemical mechanical polishing slurry to a pad; (c) rotatingthe pad; and (d) removing at least a portion of the silicon dioxidelayer by bringing the rotating pad into contact with the wafer androtating the wafer in relation to the rotating pad.